1. Field of the Invention
The present invention relates to a digital camera having an automatic white balance (AWB) adjusting function for automatically adjusting white balance and to a white balance adjusting method.
2. Description of the Related Art
In recent years, electronic cameras, that is, digital cameras, which converts a subject image by an electronic image pickup means to image data and record the image data on a recording medium, have been widely accepted. The digital camera has an adjusting function of reproducing a subject's white color called a white balance. By adjusting white balance of a captured image it is possible to obtain an image of proper color.
An ordinary digital camera has, as a white balance adjusting mode, an automatic (AWB) mode and a manual (MWB) mode. The AWB mode constitutes a mode of automatically adjusting the white balance to generally allow the white color of a captured subject to be correctly reproduced without the user's being aware of the white balance. The MWB mode constitutes a mode of allowing a specific white balance corresponding to the kinds of normal light sources used at the time of image capturing, such as sunlight, fluorescent lamp and electric bulb, to be set by manual operation of the user.
The AWB mode of automatically performing this white balance adjustment is very convenient to achieve a programmed photographing mode, or a simplified photographing mode, for simplifying image capturing operation of a digital camera.
However, the AWB mode does not work effectively under every image capturing condition and there occur cases where color reproduction of a captured subject's image is not adequate. In the case where, in particular, a subject has no white color or has extremely less white color, the white balance is sometimes upset. Stated in more detail, this applies to the case where, for example, an image is taken against a background of wholly green turf. Or even if a subject is white in color, an incorrect color decision is sometimes made depending upon the kinds of light sources at the capturing of the subject image and, in this case, the white balance is sometimes upset.
Generally, in the case of the digital camera, a reference light source under which color can be correctly reproduced is under sunlight in fair weather and has a color temperature of about 5500K. In the case where a subject image is captured in the room, it is often done under an electric bulb with a color temperature of about 3000K lower than that of the reference light source or under a light source such as a fluorescent lamp with a color temperature higher than that of the reference light source. In these situations, the white balance can be adjusted under manual mode. It is, however, not easy for the general user to achieve this and there is a high possibility that the user will perform an erroneous operation.
A system has been proposed under which comparison is made between a white balance adjustment value serving as a reference and an adjustment value under manual mode and the result of comparison is informed to the user and, if a comparison result falls out of the allowable range, a warning is issued or the capturing of an image is inhibited (Jpn. Pat. Appln. KOKAI Publications Nos. 7-162872 and 8-65689). In this system, there is no guarantee that the reference white balance adjustment value is optimal. Further, it is not possible to adjust the white balance of an image taken. Further, through the issuance of a warning or the inhibition of image capturing, there is also a problem of losing any shutter release moment.
Another, proposal has also been made under which the white balance can be manually made with monitoring an adjusted result of the white balance on the monitor (Jpn. Pat. Appln. KOKAI Publications No. 6-233333). These prior techniques perform the white balance calibration and cannot be easily used by the general user.